Hydrogen is a very important product of any petroleum refinery. Various refinery processes, such as the hydroconversion of heavy feedstocks to lower boiling products, and hydrotreating various feedstocks to remove sulfur and/or nitrogen, consume relatively large amounts of hydrogen. While other refinery processes, such as reforming, are net producers of hydrogen, refineries as a whole are typically net users of substantial amounts of hydrogen. Separate hydrogen production facilities, or the purchase of hydrogen from outside of the refinery, i add significantly to the cost of refined products. Thus, there is a substantial need for relatively inexpensive sources of hydrogen in a petroleum refinery.
Some modern complex refineries have fluid coking units. In conventional fluid coking, a petroleum feedstock is injected into a fluidized bed of hot, fine, solids and is distributed uniformly over the surfaces of the solids where it is cracked to vapors and coke. The vapors pass through a cyclone which removes most of the entrained coke particles. The vapor is then discharged into a scrubber where substantially all of the remaining solids are removed and the products are cooled to condense the heavy liquids. The resulting slurry, which usually contains from about 1 to about 3 wt. % solids is usually recycled to extinction to the coking reactor. The solids are typically coke particles.
The coke particles in the reactor vessel flow downwardly to a stripping zone at the base of the reactor where stripping steam removes interstitial product vapors from, or between, the coke particles, as well as some adsorbed liquids from the coke particles. The coke particles then flow down a stand-pipe and into a riser which leads to a burner where sufficient air is injected for burning at least a portion of the coke and heating the remainder sufficiently to satisfy the heat requirements of the coking reactor where the unburned hot coke is recycled. Net coke, above that consumed in the burner, is withdrawn as product coke.
Another type of fluid coking employs three vessels: a coking reactor, a heater, and a gasifier. Coke produced in the reactor is withdrawn and is passed to the heater where a portion of the volatile matter is removed. The coke is then passed to a gasifier where it reacts, at elevated temperatures, with air and steam to form a mixture of carbon monoxide, carbon dioxide, methane, hydrogen, nitrogen, water vapor, and hydrogen sulfide. The gas produced in the gasifier is passed to the heater to provide part of the reactor heat requirement. The remainder of the heat is supplied by circulating coke between the gasifier and the heater.
There is a need in the art for producing hydrogen in more cost efficient ways, especially if a cheap source of catalyst, such as coke from a fluid coking unit can be used.